/// The `HasManyAssociation` indicates a one-to-many connection between two
/// record types, such as each instance of the declaring record "has many"
/// instances of the other record.
///
/// For example, if your application includes authors and books, and each author
/// is assigned zero or more books, you'd declare the association this way:
///
/// ```swift
/// struct Book: TableRecord { }
/// struct Author: TableRecord {
///     static let books = hasMany(Book.self)
/// }
/// ```
///
/// A `HasManyAssociation` should be supported by an SQLite foreign key.
///
/// Foreign keys are the recommended way to declare relationships between
/// database tables because not only will SQLite guarantee the integrity of your
/// data, but GRDB will be able to use those foreign keys to automatically
/// configure your associations.
///
/// You define the foreign key when you create database tables. For example:
///
/// ```swift
/// try db.create(table: "author") { t in
///     t.autoIncrementedPrimaryKey("id")             // (1)
///     t.column("name", .text)
/// }
/// try db.create(table: "book") { t in
///     t.autoIncrementedPrimaryKey("id")
///     t.belongsTo("author", onDelete: .cascade)     // (2)
///         .notNull()                                // (3)
///     t.column("title", .text)
/// }
/// ```
///
/// 1. The author table has a primary key.
///    belongs to. This column is indexed in order to ease the selection of
///    an author's books. A foreign key is defined from `book.authorId`
///    column to `authors.id`, so that SQLite guarantees that no book refers
///    to a missing author. The `onDelete: .cascade` option has SQLite
///    automatically delete all of an author's books when that author is
///    deleted. See <https://sqlite.org/foreignkeys.html#fk_actions> for
///    more information.
/// 3. Make the `book.authorId` column not null if you want SQLite to guarantee
///    that all books have an author.
///
/// The example above uses auto-incremented primary keys. But generally
/// speaking, all primary keys are supported.
///
/// If the database schema does not define foreign keys between tables, you can
/// still use `HasManyAssociation`. But your help is needed to define the
/// missing foreign key:
///
/// ```swift
/// struct Author: TableRecord {
///     static let books = hasMany(Book.self, using: ForeignKey(...))
/// }
/// ```
public struct HasManyAssociation<Origin, Destination> {
    public var _sqlAssociation: _SQLAssociation
    
    init(
        to destinationRelation: SQLRelation,
        key: String?,
        using foreignKey: ForeignKey?)
    {
        let destinationTable = destinationRelation.source.tableName
        
        let foreignKeyCondition = SQLForeignKeyCondition(
            destinationTable: destinationTable,
            foreignKey: foreignKey,
            originIsLeft: false)
        
        let associationKey: SQLAssociationKey
        if let key {
            associationKey = .fixedPlural(key)
        } else {
            associationKey = .inflected(destinationTable)
        }
        
        _sqlAssociation = _SQLAssociation(
            key: associationKey,
            condition: .foreignKey(foreignKeyCondition),
            relation: destinationRelation,
            cardinality: .toMany)
    }
}

extension HasManyAssociation: AssociationToMany {
    public typealias OriginRowDecoder = Origin
    public typealias RowDecoder = Destination
}
